Companies' bring-your-own-device (BYOD) policies are affecting how many traditional PCs enterprises purchase and contributing to a global sales slump, IDC analysts said Tuesday.

About 25% of employees in businesses with more than 10 workers have bought the primary PC they use for work, IDC's Bob O'Donnell said, a large enough slice to meaningfully reduce corporate buying.

Because many of those PCs are already purchased systems, not machines purchased expressly for work at work, BYOD is another reason why PC shipments will slump dramatically this year, O'Donnell said in an interview.

"Employees say, 'I want to use all my own stuff,'" said O'Donnell, referring to how BYOD has shifted from a smartphone-only trend to also include worker-owned tablets and PCs. "They like their own stuff, they want to pick their own device, and choose the software they want," O'Donnell added.

The expansion of BYOD—whether approved by the business or not—and the resulting consumerization of traditional IT practices, where the company decides what employees will use, buys that hardware, and preps it with approved software, may have begun because workers pushed for the tools they wanted, but it's also been a benefit to employers, O'Donnell argued.

"There are cost-related issues involved, as well as the flexibility for employees," O'Donnell said. Chief among those "cost-related" factors is that each employee-owned PC or tablet brought to work is one less the company must buy.

"[Employee-purchased] PCs are significantly higher in number than we thought. We believed it was just 5% to 10%, but it's more than double that," O'Donnell said, citing surveys IDC has recently conducted.

The BYOD movement also means that there will be fewer PCs sold overall, as the older standard—one worker-owned machine at home, a company-bought PC at the office, for a total of two—is replaced by the one employee-bought PC for all duties. IDC's biggest rival has implied much the same: Last week Gartner predicted that by 2017, about half of the world's companies will have not only adopted BYOD programs but will have stopped providing computers and other devices to workers.

O'Donnell saw confirmation of BYOD's impact in absent PC purchases, a kind of "dark matter" that explained why enterprises have not been buying large numbers of new desktops and laptops lately.

"Windows XP will be retired from support April 8, 2014, less than a year away, but we're not really seeing a big XP to Windows 7 transition," O'Donnell said of the clock-ticking support retirement deadline for the 12-year-old XP. "There are about 200 million PCs installed with XP in business, and we expected more corporate purchases," he continued. "But that's hasn't been happening."